In the sense of "genealogical descent," this term is used by researchers in systemic-functional linguistics (SFL) to conceptualize what researchers in rhetorical genre studies (RGS) would call "genre sets" or "genre systems." In SFL, genres are classified by a scale of functional differences and similarities instead of being conceived as networked group of genres. "Agnation" maps genres along paths of similar functional relations; the hope is that making these identifications along "familial" lines will help learners deal with texts that seem further from a core sense of the genre and will help teachers discover pedagogical strategies for teaching genre and genre relations (Hyland, 2002).
Hyland, K. (2002). Genre: Language, context, and literacy. Annual Review of Applied Linguistics, 22, 113-135.
Texts can be classified using topology, for example by comparing whether they are activity-based or entity-focused, or generalized or highly-specific/particular. (Martin, 1997).
Martin, J. R. (1997). Analyzing genre: Functional parameters. In F. Christie & J.R. Martin (Eds.), Genre and institutions: Social processes in the workplace and school. Open Linguistics Series, ed. R.F. Fawcett. London: Continuum. 3-39.
Emerging Genres class, N.C. State University, Spring 2010